This proposed project will involve efforts to determine the control elements fo regulation of aminoacyl-tRNA synthetase formation in both Escherichia coli and Salmonella typhimurium, and elucidation of the exact role(s) of these enzymes in repression control of expression of the corresponding biosynthetic operons. At present, we have established that both classes of enzymes (i.e., synthetases and the respective biosynthetic enzymes) are synthesized in response to a repression control process as mediated by the supply of the cognate amino acids, functional transfer RNA and, most likely, one the biosynthetic structural gene products. The principal methods of study will involve the use of genetically based and/or physiologically imposed altered repression control responses in cells cultivated under specific growth conditions, along with purification of the cellular components presumed to be critical for control and the performance of appropriate in vitro tests of the capacities of such elements to generate the repression control signals for the aminoacyl-tRNA synthetases and the biosynthetic enzymes.